Share-Based Compensation - Weighted average assumptions used in valuing options (Details) - USD ($)	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Stock Options
Weighted-average assumptions
Dividend yield (as a percent)		1.46%
Expected life		8 years 6 months
Expected volatility (as a percent)		28.00%
Risk-free interest rate (as a percent)		2.54%
Additional disclosures
Total unrecognized compensation cost related to non vested stock option grants	$ 1,200,000
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 6 months 4 days
Total fair value of shares vested during the period	$ 799,000
Restricted Stock
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 6 months
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	104,419
Granted (in shares)	1,148
Vested (in shares)	(7,040)
Forfeited (in shares)	(1,118)
Nonvested at the end of the period (in shares)	97,409
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share)	$ 62.45
Granted (in dollars per share)	59.95
Vested (in dollars per share)	86.82
Forfeited (in dollars per share)	77.88
Nonvested at the end of the period (in dollars per share)	$ 60.48
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted	$ 1,800,000
Total fair value of shares vested during the period	$ 732,000
Restricted Stock \| Employees \| Minimum
Additional disclosures
Vesting period	4 years
Restricted Stock \| Non-employee directors \| Maximum
Additional disclosures
Vesting period	12 months
Restricted Stock Units
Additional disclosures
Weighted-average period over which unrecognized compensation cost is expected to be recognized	2 years 2 months 1 day
Restricted Stock Activity and RSUs
Nonvested at the beginning of the period (in shares)	200,540
Granted (in shares)	128,386
Forfeited (in shares)	(227)
Nonvested at the end of the period (in shares)	328,699
Weighted-Average Grant-Date Fair Value
Nonvested at the beginning of the period (in dollars per share)	$ 85.15
Granted (in dollars per share)	66.80
Forfeited (in dollars per share)	89.43
Nonvested at the end of the period (in dollars per share)	$ 77.98
Additional disclosures
Total unrecognized compensation cost related to nonvested restricted stock and RSUs granted	$ 13,500,000
Total fair value of shares vested during the period	$ 2,900,000
Target RSU award level (as a percent)	80.60%
Employee Stock Purchase Plan
Number Of Stock Issued Pursuant To Restricted Stock Units	44,599
Performance based restricted stock units
Additional disclosures
Performance period	3 years
Other Performance based restricted stock units
Additional disclosures
Performance period	1 year
Timed based restricted stock units
Additional disclosures
Performance period	4 years

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Reflects the number of stock issued pursuant to restricted stock units.

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Represents the period over which performance is measured, from grant date to maturity date, to determine the amount of the share based award.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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Unrecognized cost of unvested options awarded to employees as compensation.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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